Material that is utilized to conduct ultrasound between an electronic device and a target body is commonly referred to as an ultrasound couplant, ultrasound gel, ultrasound transmission media or acoustic transmission media. Many fluids and water-based gels have been used as ultrasound couplants over the years. Early use of mineral oil, petroleum products and cellulose compounds were replaced by gels of water and acrylic based polymers such as CARBOPOL® (a registered trademark of Noveon, Inc.). Membranes having an adhesive quality were created for use as wound dressings, drug delivery, electrical conductivity and specialized medical devices. Many of these membrane formulations were based on derivatives of acrylic acid, humectants, cross-linkers, water, natural gums and other gel formers such as derivatives of alginic acid and other polysaccharides. U.S. Pat. No. 6,039,694 to Larson et al., teaches the application solid acoustic coupling membranes, consisting of hydrogels based on co-polymers of polyacrylonitrile. Membranes and acoustic coupling sheaths of Larson et al. are lubricious on both sides and throughout, and as such, cannot be adhesively attached to the face of the transducer or to the skin of the patient. This lubricity limits the use of these hydrogel membranes since general scanning requires that the probe move freely and in contact with the acoustic coupling as it is moved back and forth over the area of interest. When the smooth lubricious surface is in contact with the skin and the probe face is moved over the lubricious outer surface of the membrane, the membranes of this invention tend to slide with the ultrasound probe requiring subsequent repositioning of the membrane. The lubricious nature of these membranes further limit their usage to horizontal surfaces since in applications such as endarterectomy and breast biopsy, the membranes slide off the irregular surfaces.
Such membranes have utility in conducting ultrasound for medical applications and non-destructive testing. These applications utilize high frequency sound, typically between 0.5 and 20 MHz. Acoustic energy at such frequencies is poorly transmitted by air and requires a coupling or conduction medium similar in acoustic properties to tissue when used in medical diagnostic imaging and Non-Destructive Testing (NDT) applications. These acoustic coupling media have been commonly thick fluids, gels, or a media in solid form to transfer the acoustic energy between the target object and electronic devices. Ultrasound coupling media displace air and fills contours between the piezoelectric transducer or “eye” of the instrument, referred to in the industry as an ultrasound probe, which converts energy between electrical and acoustic, and the object of interest into and from which the acoustic energy is directed. This media by nature of its physical and acoustic properties, serves as an ultrasound acoustic coupler between the object and the electronic transducer, thereby acoustically joining the two, so that the ultrasound based information developed can freely pass back and forth between the object and the transducer.
Of recent development are biocompatible adhesive hydrogel membranes that are produced from formulations of polymers such as polyvinyl alcohol, polyvinylpyrrilidone (PVP), and polyethylene oxide (PEO) by cross-linking initiated by high-energy sources such as e-beam, gamma and ultraviolet radiation. The membranes are produced by casting polymer blends on a suitable backer, such as a polyethylene sheet, followed by high-energy irradiation, which cross-links the polymer blend to form a cohesive membrane that is flexible and adhesive throughout. Such cross-linked membranes are commonly formulated to be adhesive and hydrophilic, thus providing utility as wound dressings, cosmetic facial masks, and when impregnated with drugs; for example, hormones, anti-infective agents, analgesics, therapeutics and local anesthetics, can be used as agents for drug delivery. By addition of certain salts, i.e. potassium chloride, sodium chloride and magnesium acetate to the base formulations, the membranes can be made conductive and fabricated to form devices for physiological monitoring.
Such formulations also render these adhesive hydrogel membranes acoustically conductive, providing acceptable low levels of ultrasound impedance and artifact with excellent ultrasound transmission, thus creating membranes that can perform as solid couplants, suitable for medical ultrasound imaging and ultrasound based therapy applications in place of gels, thickened liquids and couplants as are known in the art.
Since many hydrogel membranes are commonly adhesive on both sides and throughout, the ease and range of use of these materials is generally limited to static applications such as fetal monitoring or those in which the adhesive membrane is enclosed in a protective cover as a means of conducting acoustic energy from the active face of an ultrasound probe through the cover and into an external couplant. The characteristic of adhesivity on all faces and throughout the hydrogel membrane limits its use with ultrasound probes by restricting free motion of the probe over the surface of the membrane such as when a membrane is placed on the skin as a coupling media or when attempting insertion of a membrane covered probe face into protective covers as is common procedure when used during surgery and intracavity examinations.
An advantage of the device of the present invention is that the above impediments caused by the adhesive nature of the external surface, as is present when its use is so configured such that the membrane is attached to the active face of an ultrasound probe or positioned on skin for ultrasound imaging and ultrasound based therapies, are substantially eliminated.